CHAPTER 9 DATA ACQUISITION

Transcript
CHAPTER 9 DATA ACQUISITION

DATA ACQUISITION
Today’s Topics
•
Define DAQ and DAQ systems
•
Signals (digital and analogue types)
•
Transducers
•
Signal Conditioning
- Importance of grounding
- Differential, Referenced and nonferenced single
ended signals
- A to D conversion considerations (resolution,
device and signal range, sampling rate)
•
In Class Example – Signal input range calculation
•
Assign homework and begin in class
DATA ACQUISITION - DEFINED
• DATA ACQUISITION (DAQ) is the measurement
or generation of physical signals.
• DAQ systems typically consists of five elements:
1. Signals
2. Transducers
3. Signal-conditioning hardware
4. DAQ device or module
5. Application software
TYPICAL DAQ SYSTEM ELEMENTS
SIGNALS
• Signals are physical quantities that are functions
of an independent variable (such as time) and
contain information about a natural
phenomenon
• Two types of signals may be defined; digital and
analogue.
- Digital signals provide information regarding
the voltage state (typically hi or low) and/or
the rate of change of these states
- An analogue signal typically provides voltage
level, shape or frequency content information.
Signal types and classes
• Two types of Digital signals:
- on-off
• Three types of analogue
- DC
- AC
•Total of 5 signal classes
- pulse train,
- frequency domain
• ADC - analogue to digital converter, which converts the analogue signal
into a digital signal which can be read by a computer
• DAC - digital to analogue converter, which converts a digital signal to an
analogue signal
• TTL - transistor to transistor logic
TRANSDUCERS
• A transducer is a device which converts a physical
phenomenon into a measurable electrical signal.
SIGNAL CONDITIONING
Transducer output is usually computer ready and must be
conditioned, either using hardware or software:
Transducer excitation – use of an external voltage or
current to excite the transducer (ex: strain gauges)
•Linearization – creation of a linear relationship between
the transducer output and the physical phenomenon being
measured
•Isolation - isolation of the transducer signal from the
computer and other transducers
•Filtering - conditioning of a signal to reduce unwanted
components (i.e., noise, high or low frequency components,
etc.)
•Amplification - increace of the output signal of the
transducer to increace accuracy and signal to noise ratio
LABVIEW SCXI
SIGNAL CONDITIONING
• SCXI (Signal Conditioning Xtensions for
Instrumentation) is a hardware device
designed to condition low level signals in a
noisy environment within an external
chassis located near the sensor.
• Some signal conditioning, such as
linearization and filtering, can often be
performed using software, and LabVIEW
provides several VIs for these purposes.
SIGNAL GROUNDING
• Two types of signal sources
- grounded: signals which are referenced
to a system ground
ex:110V outlets, signal generators, power
supplies
- floating: signals which are not connected
to an absolute reference
ex: battery powered sources, thermocouples
transformers
NOTES ON
MEASUREMENT SYSTEMS
A measurement system can be placed into on of three categories:
1. Differential:
- Neither the positive or negative terminal of the source or
transducer is connected to ground.
- The system only measures the difference between the terminals,
and rejects other voltages in the system. - It is therefore
preferred.
2. Referenced single ended (RSE)
- Measures a signal with respect to the system ground.
- Use if more channels are required.
3. Non-referenced single ended (NRSE)
- all measurements are made with respect to a common reference
other than the system ground.
ANALOGUE TO DIGITAL
CONVERSION CONSIDERATIONS
The quality of the analogue to digital conversion is
dependent on the following four parameters:
1. Resolution
2. Device range
3. Signal input range
4. Sampling rate
These will be set using hardware (board switches)
or software (Measurement and Automation
Explorer in Labview)
1. RESOLUTION
• Resolution: The number of bits used to
represent the analogue signal
16-bit
The above example shows the difference
between 3 (23 =8 divisions) and 16 bit (216 =
65,536 divisions) resolution
2. DEVICE RANGE
• device range: minimum and maximum
analogue signal levels that the ADC can
convert.
• The device range should be matched to
the range of the analogue input signal
to best take advantage of the available
resolution!
2. DEVICE RANGE (cont’s)
• Example: If a 3-bit ADC (having 8 divisions) is used over
a range of 0 to 10 volts, voltage changes of 1.25V can be
measured.
However, if the range is increased to -10 to 10V, then the
smallest voltage change which can be measured rises to
2.5V
3. SIGNAL INPUT RANGE
• signal input range: the maximum and minimum value of
the signal being measured.
• The closer the signal input range is to the incoming
analogue signal max and min, the more digital divisions
will be available to the ADC to represent the signal
SMALLEST DETECTABLE
VOLTAGE CHANGE
• Determined by the resolution and range of a DAQ
device and the signal input range.
• This change in voltage represents 1 least
significant bit (LSB) of the digital value and is often
called the code width.
• This smallest code width, Vcw is calculated as
follows (Where the resolution is given in bits)
Example: a 12 bit DAQ device with a 0 to 10V range can
detect a 2.4 mV change.
• Unipolar signals range from 0 to a positive value
• bipolar signals range from a negative to a positive value.
4. SAMPLING RATE
• sampling rate: the rate at which the DAQ device
samples an incoming analogue signal
• Determines how often an analogue to digital conversion
takes place.
• Computing the proper sampling rate requires knowledge
of the max frequency of the incoming signal and the
accuracy required for digital representation.
• In general, a fast sampling rate acquires more points per
unit time and results in a better representation of the
incoming signal.